Problem: Which of these numbers is composite? ${13,\ 17,\ 66,\ 71,\ 89}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 13, 17, 71, and 89 each have only two factors. The factors of 66 are 1, 2, 3, 6, 11, 22, 33, and 66. Thus, 66 is the composite number.